Reduced S/D contact resistance of III-V MOSFET using low temperature metal-induced crystallization of n+ Ge

ABSTRACT

Embodiments of this invention provide a method to fabricate an electrical contact. The method includes providing a substrate of a compound Group III-V semiconductor material having at least one electrically conducting doped region adjacent to a surface of the substrate. The method further includes fabricating the electrical contact to the at least one electrically conducting doped region by depositing a single crystal layer of germanium over the surface of the substrate so as to at least partially overlie the at least one electrically conducting doped region, converting the single crystal layer of germanium into a layer of amorphous germanium by implanting a dopant, forming a metal layer over exposed surfaces of the amorphous germanium layer, and performing a metal-induced crystallization (MIC) process on the amorphous germanium layer having the overlying metal layer to convert the amorphous germanium layer to a crystalline germanium layer and to activate the implanted dopant. The electrical contact can be a source or a drain contact of a transistor.

TECHNICAL FIELD

The exemplary embodiments of this invention relate generally tosemiconductor devices and to procedures to fabricate semiconductordevices and, more specifically, relate to transistors, such as MOSFETs,comprised of Group III-V semiconductor material having amorphousGermanium contacts.

BACKGROUND

The use of metal-induced dopant activation (MIDA) in amorphous Germanium(α-Ge) (Germanium is a Group IV element) is a known process. Forexample, reference can be made to Jin-Hong Park, M. Tada, D. Kuzum, P.Kapur, H-.Y. Yu, H-.S. Philip Wong, and K. C. Saraswat, “Low Temperature(≦380° C.) and High Performance Ge CMOS Technology with NovelSource/Drain by Metal-Induced Dopants Activation and High-K/Metal GateStack for Monolithic 3D Integration,” IEEE International ElectronDevices Meeting (IEDM) 2008 Technical Digest, pp. 389-392, SanFrancisco, Calif., Dec. 15-17, 2008; and Jin-Hong Park, D. Kuzum, M.Tada and K. C. Saraswat, “High Performance Germanium N⁺/P and P³⁰/Njunction diodes formed at Low Temperature (<380° C.) using Metal-InducedDopant Activation”, Applied Physics Letters 93, 193507 (2008). Referencecan also be made to Jin-Hong Park, M. Tada, Woo-Shik Jung, H-.S. PhilipWong, and K. C. Saraswat, “Metal-induced dopant (boron and phosphorus)activation process in amorphous germanium for monolithicthree-dimensional integration”, Journal of Applied Physics 106, 074510(2009).

FIG. 1 herein reproduces FIG. 3( b) from the Park et al. Applied PhysicsLetters 93 publication and shows SIMS (secondary ion mass spectrometer)and SRP (spreading resistance profiling) data (carrier concentration asa function of depth) of an N⁺/P junction on epi-Ge annealed at 360° C.for 10 minutes with Co and at 600° C. for 1 minute without Co. TheFigure illustrates the distinction between P implantation and Co-inducedcrystallization.

A problem that arises when considering the fabrication of transistorsusing a Group III-V compound semiconductor (composed of elements fromGroup III and Group V of the periodic table of elements) is that due atleast to a reliable silicide process, the self-aligned integration of aGroup III-V transistor (e.g., a metal oxide semiconductor field effecttransistor or MOSFET) is difficult to accomplish. This difficulty canaffect the ability to fabricate low resistance electrical contacts tothe source and drain regions of the transistor.

SUMMARY

The foregoing and other problems are overcome, and other advantages arerealized, in accordance with the exemplary embodiments of thisinvention.

In one aspect thereof the embodiments of this invention provide a methodto fabricate an electrical contact, comprising providing a substratecomprised of a compound Group III-V semiconductor material having atleast one electrically conducting doped region adjacent to a surface ofthe substrate; and fabricating the electrical contact to the at leastone electrically conducting doped region by depositing a single crystallayer of germanium over the surface of the substrate so as to at leastpartially overlie the at least one electrically conducting doped region,converting the single crystal layer of germanium into a layer ofamorphous germanium by implanting a dopant, forming a metal layer overexposed surfaces of the amorphous germanium layer, and performing ametal-induced crystallization (MIC) process on the amorphous germaniumlayer having the overlying metal layer to convert the amorphousgermanium layer to a crystalline germanium layer and to activate theimplanted dopant.

In another aspect thereof the embodiments of this invention provide amethod to fabricate a semiconductor device, comprising providing a GroupIII-V substrate having a source region and a drain region that arespaced apart adjacent to a surface of the substrate and that define achannel there between, and disposed over the channel a gate stackstructure contained within a dielectric material; and fabricating asource contact and a drain contact by selectively depositing a singlecrystal layer of germanium over the surface of the substrate so as tooverlie the source region and the drain region but not the dielectricmaterial, implanting the crystalline layer of germanium with a dopantand converting the crystalline layer of germanium to a doped amorphousgermanium layer, forming an overlying metal layer over exposed surfacesof the doped amorphous germanium layer, applying heat to convert theamorphous germanium layer having the overlying metal layer to acrystalline germanium layer and to activate the implanted dopant, andremoving the overlying metal layer and the underlying dielectricmaterial from the gate stack structure so as leave a source spacerbetween the crystalline germanium layer that overlies the source regionand a drain spacer between the crystalline germanium layer that overliesthe drain region.

In a further aspect thereof the embodiments of this invention provide atransistor that comprises a compound Group III-V substrate having asource region and a drain region that are spaced apart adjacent to asurface of the substrate and that define a channel there between; a gatestack structure disposed over the channel; electrically insulatingsource and drain spacers disposed at least partially over the source anddrain regions, respectively, and abutting the gate stack structure; anda source contact and a drain contact at least partially overlying thesource region and the drain region, respectively, and separated from thegate stack structure by the source and drain spacers, respectively, saidsource and drain contacts each comprised of a layer of crystalline dopedgermanium having an overlying layer of metal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a graph showing shows SIMS (secondary ion mass spectrometer)and SRP (spreading resistance profiling) data (carrier concentration asa function of depth) of an N⁺/P junction on epi-Ge annealed at 360° C.for 10 minutes with Co and at 600° C. for 1 minute without Co.

FIGS. 2A-2E, collectively referred to as FIG. 2, illustrate enlargedcross-sectional views of a Group III-V MOSFET at different fabricationpoints in accordance with the embodiments of this invention.

FIGS. 3 and 4 are each a process flow diagram in accordance with theexemplary embodiments of this invention.

DETAILED DESCRIPTION

FIG. 2A depicts a starting structure 1 for a Group III-V electronicdevice, such as a MOSFET. The starting structure 1 has beenpre-fabricated using a Group III-V substrate 10. The Group III-Vsubstrate 10 can be a binary Group III-V material, such as GaAs or atertiary Group III-V material such as GaAlAs, or a quarternary GroupIII-V material such as a GaAlAsP, as non-limiting examples. The GroupIII-V material substrate 10 has formed therein an N⁺ source 12 regionand an N⁺ drain 14 region. The N⁺ source 12 region and the N⁺ drain 14region may be formed by ion implanting a suitable N⁺ species such as Si.A channel, also referred to herein as a gate region 15, will existbetween the N⁺ source 12 region and the N⁺ drain 14 region (the channelexists in the Group III-V material). The gate region 15 may have alength of, by example, 30 nanometers or less. Over the gate region 15 isformed a gate stack structure 2 that includes an electrically insulatinghigh dielectric layer (high-k) layer 16. The high-k layer 16 may be alayer of, by example, hafnium dioxide (HfO₂), zirconium dioxide (ZrO₂),or titanium dioxide (TiO₂), all of which inherently have a dielectricconstant or “k” above 3.9, the dielectric constant of Silicon Dioxide(SiO₂). The high-k layer 16 may have a thickness in the range of about 1nanometer to about 5 nanometers, with 3 nanometers being a typicaluseful value. The gate stack structure 2 further includes a layer ofgate metallization 18, such as a layer of Tungsten, or Titanium Nitride(TiN), or Tantalum Nitride (TaN), or polysilicion, as severalnon-limiting examples. The gate metallization layer 18 can have athickness of, for example, about 30 nanometers. Overlying the gate stackstructure 2 is a dielectric layer forming an electrically insulatinglayer 20 of, for example, Silicon Dioxide (SiO₂) or Silicon Nitride(SiN). Those portions of the insulating layer 20 that are disposed alongthe sides of the gate stack structure 2 will subsequently function asgate spacers.

FIG. 2B illustrates a first intermediate transistor structure based onthe starting structure 1. In FIG. 2B it can be seen that a layer 22 ofGe is grown over the N⁺ source 12 region and the N⁺ drain 14 region,i.e., on the Group III-V substrate. The Ge layer 22 can be grown at atemperature that exceeds about 450° C. and as formed is substantially asingle crystal material and can have a thickness of, for example, about50 nanometers. The Ge layer 22 can be formed by any suitable growthprocess such as metal-organic chemical vapor deposition (MOCVD) or highvacuum chemical vapor deposition (CVD). Due to the propensity of Ge topreferentially nucleate and grow only on the Group III-V substratematerial, and not on the SiO₂ or SiN insulator layer 20, a maskingoperation is not required. In effect, the growth of the GE layer 22 maybe viewed as a self-aligned growth process.

This aspect of the invention thus selectively grows Ge on a III-Vsubstrate patterned with, for example, SiO_(x), SiN_(x) or Al₂O₃. Whileit is possible that there may be some minor nucleation of Ge on thedielectric surface as well this can be inhibited by, for example,increasing the growth temperature or by introducing HCl gas during theGe growth in CVD.

FIG. 2C illustrates a second intermediate transistor structure based onthe first intermediate transistor structure of FIG. 2B. FIG. 2C showsthe performance of a Phosphorous ion implant (P I/I). The dosage can beabout 10¹⁵ cm² that is applied at an energy of about 10 key (or less).The P I/I converts the single crystal Ge layer 22 to an amorphous Ge(α-Ge) layer 22A.

FIG. 2D illustrates a third intermediate transistor structure based onthe second intermediate transistor structure of FIG. 2C. FIG. 2D showsthe blanket deposition of a metal layer 24 containing, for example,Nickel or Cobalt. The metal layer 24 may have a thickness of about 10nanometers, and can be formed using any suitable metal depositionprocess such as e-beam deposition or sputtering.

FIG. 2E illustrates a final transistor structure (although otheradditional conventional processing could still be performed) that isbased on the third intermediate transistor structure of FIG. 2D. Toarrive at the final transistor structure shown in FIG. 2E ametal-induced crystallization (MIC) process is performed to drive in andactivate the P dopant implanted in FIG. 2C, thereby converting the α-Gelayer 22A to a crystallized (a polycrystalline) n⁺ Ge layer 22B. The MICprocess is preferably a relatively low temperature process that iscarried out at a temperature of about 400° C. or less (e.g., in a rangeof about 150° C. to about 380° C. for a period of about 1 hour to about4 hours). A wet etch process (e.g., one using HCl) can be used then toremove that portion of the metallization layer 24 that overlies the SiO₂or SiN insulator disposed over the top of the gate structure 2, followedby the removal of the underlying SiO₂ or SiN insulator material, therebyexposing the Tungsten layer 18 that forms the gate contact. Theunderlying SiO₂ or SiN insulator material can be removed using, forexample, any suitable etching process. The resulting final transistorstructure is thus characterized by high conductivity (e.g., in range ofabout 1e-6 ohm-cm to about 1 ohm-cm) n⁺ crystalline Ge S/D contactshaving an overlayer 24A of, e.g., NiGe or CoGe, where the S/D contactsare insulated from the gate stack materials by the intervening SiO₂ orSiN spacers 20A, 20B that abut side walls of the gate stack structure 2.The metal-induced dopant activation (MIDA) process beneficially providesthe crystallized n⁺ Ge layer 22B, and what may be viewed as a NiGe orCoGe silicide process, and which thus forms the basis of the formationof the low temperature budget (lower annealing temperature) and lowresistivity S/D contacts The dopant concentration in the n+ Ge S/Dcontacts 22B may be about 5×10¹⁹ atoms/cm³ or greater.

It should be realized that the embodiments of this invention may beimplemented by using materials other than those expressly mentionedabove, and/or by using different material thicknesses, and/or by usingdifferent temperatures and times, and/or by using different dopantconcentrations and ion implant species (e.g., Boron) and implantparameters. For example, in other embodiments a metal such as Al may beused instead of Ni or Co, with corresponding adjustments made to MICtemperatures and times so as to achieve the desired crystalline grainsize and dopant activation. Reference in this regard can be made to, forexample, Shu Hu, Ann F. Marshall, and Paul C. McIntyre,“Interface-controlled layer exchange in metal-induced crystallization ofgermanium thin films”, Applied Physics Letters 97, 082104 (2010). Theseauthors report metal (Al)-induced crystallization to form poly-Ge thinfilms on glass and polymer substrates. To avoid a disordered mixture ofAl and crystalline Ge the α-Ge surface is intentionally oxidized byflowing ozone gas at room temperature to form a very thin GeO_(x) layerprior to Al deposition, followed by an anneal of the bilayers in vacuumat 250° C. for one hour. The same or a similar procedure can be usedduring fabrication of the final transistor structure shown in FIG. 2E.

Further by example, in other embodiments a transistor having p⁺ sourceand drain regions 12 and 14 may be formed, and in still otherembodiments the electronic device that is fabricated to have improvedcontacts may be other than a transistor (e.g., a light-emitting or aphotodiode or a solar cell).

FIG. 3 is a process flow diagram in accordance with the exemplaryembodiments of this invention. More specifically, FIG. 3 shows a processto fabricate an electrical contact. In Block 3A there is a step ofproviding a substrate comprised of a compound Group III-V semiconductormaterial having at least one electrically conducting doped regionadjacent to a surface of the substrate. In Block 3B there is a step offabricating the electrical contact to the at least one electricallyconducting doped region by depositing a single crystal layer ofgermanium over the surface of the substrate so as to at least partiallyoverlie the at least one electrically conducting doped region. In Block3C the electrical contact is further fabricated by converting the singlecrystal layer of germanium into a layer of amorphous germanium byimplanting a dopant. In Block 3D the electrical contact is furtherfabricated by forming a metal layer over exposed surfaces of theamorphous germanium layer. In Block 3E the electrical contact is furtherfabricated by performing a metal-induced crystallization process on theamorphous germanium layer having the overlying metal layer to convertthe amorphous germanium layer to a crystalline germanium layer and toactivate the implanted dopant.

FIG. 4 is a process flow diagram further in accordance with theexemplary embodiments of this invention. More specifically, FIG. 4 showsa process to fabricate a semiconductor device. In Block 4A there is astep of providing a Group III-V substrate having a source region and adrain region that are spaced apart adjacent to a surface of thesubstrate and that define a channel there between, and disposed over thechannel a gate stack structure contained within a dielectric material,In Block 4B there is a step of fabricating a source contact and a draincontact by selectively depositing a single crystal layer of germaniumover the surface of the substrate so as to overlie the source region andthe drain region but not the dielectric material. In Block 4C the sourcecontact and the drain contact are further fabricated by implanting thecrystalline layer of germanium with a dopant and converting thecrystalline layer of germanium to a doped amorphous germanium layer. InBlock 4D the source contact and the drain contact are further fabricatedby forming an overlying metal layer over exposed surfaces of the dopedamorphous germanium layer. In Block 4E the source contact and the draincontact are further fabricated by applying heat to convert the amorphousgermanium layer having the overlying metal layer to a crystallinegermanium layer and to activate the implanted dopant. In Block 4F thesource contact and the drain contact are further fabricated by removingthe overlying metal layer, such as Ni or Co or Al, and the underlyingdielectric material from the gate stack structure so as leave a sourcespacer between the crystalline germanium layer that overlies the sourceregion and a drain spacer between the crystalline germanium layer thatoverlies the drain region.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

As such, various modifications and adaptations may become apparent tothose skilled in the relevant arts in view of the foregoing description,when read in conjunction with the accompanying drawings and the appendedclaims. As but a few some examples, and as was indicated previously, theuse of other similar or equivalent semiconductor materials, dopants,etchants, process chambers and the like may be used by those skilled inthe art. However, all such and similar modifications of the teachings ofthis invention will still fall within the scope of this invention.

What is claimed is:
 1. A method to fabricate an electrical contact,comprising: providing a substrate comprised of a compound Group III-Vsemiconductor material having at least one electrically conducting dopedregion adjacent to a surface of the substrate; and fabricating theelectrical contact to the at least one electrically conducting dopedregion by depositing a single crystal layer of germanium over thesurface of the substrate so as to at least partially overlie the atleast one electrically conducting doped region, converting the singlecrystal layer of germanium into a layer of amorphous germanium byimplanting a dopant, forming a metal layer over exposed surfaces of theamorphous germanium layer, and performing a metal-inducedcrystallization process on the amorphous germanium layer having theoverlying metal layer to convert the amorphous germanium layer to acrystalline germanium layer and to activate the implanted dopant.
 2. Themethod of claim 1, where the dopant is comprised of phosphorous orboron.
 3. The method of claim 1, where the metal layer is comprised ofnickel or cobalt or aluminum.
 4. The method of claim 1, where themetal-induced crystallization process occurs at a temperature of about400° C. or less.
 5. A method to fabricate a semiconductor device,comprising: providing a Group III-V substrate having a source region anda drain region that are spaced apart adjacent to a surface of thesubstrate and that define a channel there between, and disposed over thechannel a gate stack structure contained within a dielectric material;and fabricating a source contact and a drain contact by selectivelydepositing a single crystal layer of germanium over the surface of thesubstrate so as to overlie the source region and the drain region butnot the dielectric material, implanting the crystalline layer ofgermanium with a dopant and converting the crystalline layer ofgermanium to a doped amorphous germanium layer, forming an overlyingmetal layer over exposed surfaces of the doped amorphous germaniumlayer, applying heat to convert the amorphous germanium layer having theoverlying metal layer to a crystalline germanium layer and to activatethe implanted dopant, and removing the overlying metal layer and theunderlying dielectric material from the gate stack structure so as leavea source spacer between the crystalline germanium layer that overliesthe source region and a drain spacer between the crystalline germaniumlayer that overlies the drain region.
 6. The method of claim 5, wherethe dielectric material is comprised of one of silicon dioxide orsilicon nitride, and where selectively depositing blanket depositsgermanium so as to preferentially nucleate only on the Group III-Vsubstrate and not the dielectric layer.
 7. The method of claim 5, wherethe dopant is comprised of phosphorous.
 8. The method of claim 5, wherethe dopant is comprised of boron.
 9. The method of claim 5, where themetal layer is comprised of nickel.
 10. The method of claim 5, where themetal layer is comprised of cobalt.
 11. The method of claim 5, where themetal layer is comprised of aluminum.
 12. The method of claim 5, whereapplying heat applies heat sufficient to perform metal inducedcrystallization (MIC) of the amorphous germanium layer.
 13. The methodof claim 12, where applying heat occurs at a temperature of about 400°C. or less
 14. The method of claim 5, where the metal layer is formed bya blanket deposition process so as to cover the exposed surfaces of thedoped amorphous germanium layer and the dielectric material thatcontains the gate stack structure, and where removing the metal layerfrom the gate stack structure comprises removing only a portion of thelayer of metal that covers the dielectric material and not a portion ofthe layer of deposited metal that covers the doped amorphous germaniumlayer.
 15. The method of claim 14, where the gate stack structure iscomprised of a layer of high dielectric constant material and a layer ofgate metallization overlying the layer of high dielectric constantmaterial and where removing the metal layer and the underlyingdielectric material from the gate stack structure comprises exposing thelayer of gate metallization.
 16. A method to fabricate a semiconductordevice, comprising: providing a Group III-V substrate having a sourceregion and a drain region that are spaced apart adjacent to a surface ofthe substrate and contained within the Group III-V substrate, the sourceregion and the drain region defining a channel there between anddisposed over the channel a gate stack structure contained within adielectric material; and fabricating a source contact and a draincontact by selectively depositing a single crystal layer of germaniumover the surface of the substrate so as to overlie the source region andthe drain region but not the dielectric material, implanting thecrystalline layer of germanium with a dopant and converting thecrystalline layer of germanium to a doped amorphous germanium layer,forming an overlying metal layer over exposed surfaces of the dopedamorphous germanium layer, applying heat to convert the amorphousgermanium layer having the overlying metal layer to a crystallinegermanium layer and to activate the implanted dopant, where the heat issufficient to perform metal induced crystallization (MIC) of theamorphous germanium layer; and removing the overlying metal layer andthe underlying dielectric material from the gate stack structure so asleave a source spacer between the crystalline germanium layer thatoverlies the source region and a drain spacer between the crystallinegermanium layer that overlies the drain region.
 17. The method of claim16, where the dielectric material is comprised of one of silicon dioxideor silicon nitride, and where selectively depositing blanket depositsgermanium so as to preferentially nucleate only on the Group III-Vsubstrate and not the dielectric layer.
 18. The method of claim 16,where the dopant is comprised of one of phosphorous or boron, and wherethe metal layer is comprised of one of nickel, cobalt or aluminum. 19.The method of claim 16, where applying heat occurs at a temperature ofabout 400° C. or less.
 20. The method of claim 16, where the metal layeris formed by a blanket deposition process so as to cover the exposedsurfaces of the doped amorphous germanium layer and the dielectricmaterial that contains the gate stack structure, and where removing themetal layer from the gate stack structure comprises removing only aportion of the layer of metal that covers the dielectric material andnot a portion of the layer of deposited metal that covers the dopedamorphous germanium layer.
 21. The method of claim 20, where the gatestack structure is comprised of a layer of high dielectric constantmaterial and a layer of gate metallization overlying the layer of highdielectric constant material and where removing the metal layer and theunderlying dielectric material from the gate stack structure comprisesexposing the layer of gate metallization.